1. Field of the Invention
This invention relates to a vacuum cleaner.
2. Description of the Prior Art
A vacuum cleaner is known which comprises a dust sensor provided in a passage for sucking air and a controlling circuit for controlling the suction force in accordance with a detection signal from the dust sensor.
Such prior art vacuum cleaner is described in U.S. Pat. No. 4,801.082. However, in the above-mentioned prior art vacuum cleaner, the suction force is changed stepwise between high and low values. Thus, there is a drawback that the suction force is not proportionally varied with the degree of uncleanliness of the floor, i.e., the amount of dust to be sucked so that the suction force increases excessively if uncleanliness exceeds a given level.